The Romans did have problems with blocked sewers but much of the detritus that makes up the fatberg is modern - wet wipes, sanitary napkins, cotton buds and so on. Also, there are some other points to consider here, namely:
The main Roman sewer, the Cloaca Maxima, was originally built to drain land between the seven hills. It's primary function was not to ...
The only way to mesure decadence is fail. However a non-decadent society, so the two parameters are not entirely linked.
No, because a decadent society is not well defined: it consists in two things:
A society getting less focused on the initial values that it followed earlier
A society that gets more and more problem (eg: ...